JP2006287219A - Thin film transistor for imaging system and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin film transistor which is used in a detector of an imaging system and reduces electronic noise, and a manufacturing method thereof. <P>SOLUTION: A detector (22) comprises an electrode consisting of a first layer of a conductive material, a readout line consisting of a second layer of a conductive material, and a via electrically connecting the readout line and the electrode. In one embodiment, the detector includes a source electrode (38) and a drain electrode (40) which consist of a first layer of a conductive material and a data line (44) which consists of a second layer of a conductive material so that the source electrode (38) and drain electrode (40) are vertically offset from the data line (44). Alternatively, in another embodiment, the detector includes a gate electrode (46) consisting of a first layer of a conductive material and a scan line (58) consisting of a second layer of a conductive material so that the gate electrode (46) is vertically offset from the scan line (58). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates generally to imaging systems. Specifically, the present invention relates to a thin film transistor used for a detector of such an imaging system and a method for manufacturing the same.

  Non-invasive imaging broadly includes techniques for forming images of internal structures or internal regions of a human body or object that are not accessible by visual inspection. For example, non-invasive imaging techniques are widely used in the industrial field for inspecting the internal structure of parts, and in the security field for inspecting contents such as packaging and clothing. However, one of the best known uses of non-invasive imaging is in medical technology, which uses these techniques to form images of internal organs and / or bones of an invisible patient .

  One class of non-invasive imaging techniques that can be used in these various fields is based on X-ray transmission differences through a patient or object. In a medical environment, a simple X-ray imaging technique is to generate X-rays using an X-ray tube or other radiation source and irradiate the imaging space where the imaged portion of the patient is placed with X-rays. Good. As x-rays pass through the patient, they are attenuated based on the composition of the tissue through which the x-rays pass. The attenuated x-rays then enter the detector, which converts the x-rays into signals and processes these signals to produce an image of the portion of the patient through which the x-rays pass based on the x-ray attenuation. Can be formed. Typically, in the X-ray detection step, a scintillator that generates optical photons when X-rays enter and an array of photosensitive elements that generate electrical signals based on the number of detected photons are used.

Some x-ray techniques use very low x-ray flux so that patient irradiation can be extended. For example, fluoroscopic techniques are widely used to monitor ongoing procedures or conditions such as insertion of a catheter or probe into a patient's circulatory system. Such fluoroscopic techniques typically acquire a large number of images, and these images can be displayed continuously to show the movement of the imaging area in real time or near real time.
US Pat. No. 6,362,028

  However, the fluoroscopic technique is similar to other low X-ray flux imaging techniques, but the image quality may be deteriorated because the X-ray signal is weaker than the electronic noise belonging to the detector. As a result, it is typically desirable to increase the efficiency of the detection process, such as by reducing electronic noise belonging to the detector. For example, various aspects of the thin film transistor (TFT) used to read the detector element can contribute to the overall electronic noise. For example, the source and drain of a TFT may be formed of the same material layer as the counterpart data line to which the source and drain are connected. Typically, however, data lines have different electrical characteristics than the source and drain, and therefore configurations that enhance data line performance may impair source and drain performance, and vice versa. For example, it may be desirable for the data line to be relatively thick to reduce resistance. However, if thick source and drain lines are cut down during the wet etching performed at the time of TFT formation, the channel length increases. In general, the longer the channel, the greater the problems associated with electronic noise, because electronic noise is a function of charge trapping and charge trapping is a function of channel length. In addition, the “on” resistance of the TFT is also a function of the channel length, and the pixel readout speed decreases as the channel length increases. As a result, a shorter channel length is generally desirable to improve TFT performance. Because of these conflicting aspects, data lines and / or source and drain electrodes that are formed of the same material layer (and therefore have the same thickness) and / or source and drain electrodes may not have optimal performance.

  Similarly, scan lines connected to TFT gates can also affect detector performance. Specifically, the scan line and the gate are typically formed of the same material layer. However, as with the data line, it may be desirable that the scan line is thicker in order to reduce the resistance of the scan line. However, the resulting thick gate electrode increases the edge step of the gate electrode, which can cause leakage current. Therefore, if the electrical characteristics of the readout lines (that is, the scanning lines and the data lines) are increased, the electrodes formed of the same material layer are adversely affected, and the performance of the detector may be unintentionally impaired. is there.

  Accordingly, there is a need for a thin film transistor that addresses some or all of the problems discussed above.

  In one aspect of the present technique, a detector is provided. The detector includes an electrode formed of a first layer of conductive material, a readout line formed of a second layer of conductive material, and a via that electrically connects the readout line and the electrode. It is out.

  In another aspect of the present technique, a detector for an imaging system is provided. The detector includes a plurality of detector elements each having a thin film transistor. The thin film transistor includes a source electrode and a drain electrode formed of a first layer of a conductive material, a data line formed of a second layer of a conductive material, and a via that electrically connects the data line and the drain electrode. Including.

  In yet another aspect of the present technique, a detector for an imaging system is provided. The detector includes a plurality of detector elements each including a thin film transistor. The thin film transistor includes a gate electrode formed of a first layer of conductive material, a scan line formed of a second layer of conductive material, and a via that electrically connects the scan line and the gate electrode. It is out.

  In yet another aspect of the present technique, a detector for an imaging system is provided. The detector includes a plurality of detector elements each including a thin film transistor. The thin film transistor was formed with a gate electrode formed with a first layer of conductive material, a source electrode and a drain electrode formed with a second layer of conductive material, and a third layer of conductive material A scan line, a first via electrically connecting the scan line and the gate electrode, a data line formed of a fourth layer of a conductive material, and a second electrically connecting the data line and the drain electrode. Including vias.

  These and other features, aspects and advantages of the present invention will become more fully understood when the following detailed description is read in conjunction with the accompanying drawings. In the accompanying drawings, like reference numerals denote like members throughout the drawings.

  FIG. 1 is a diagram of an x-ray imaging system indicated generally by the reference numeral 10. In the illustrated embodiment, the x-ray imaging system 10 is designed to acquire and process image data according to the techniques of the present invention, as will be described in detail below. X-ray imaging system 10 includes an X-ray source 12 disposed adjacent to a collimator 14. In one embodiment, the X-ray source 12 is a low X-ray flux source and is used in a low X-ray flux imaging technique such as a fluoroscopic technique. As will be appreciated by those skilled in the art, x-ray flux represents the amount of x-rays emitted by an x-ray source per unit time. The collimator 14 passes the x-ray radiation stream 16 through an area where a target 18 such as a patient is located. Some of the radiation is attenuated by the target 18. This attenuated radiation 20 is incident on a detector 22 such as a fluoroscopic detector. As will be described later, the detector 22 converts X-ray photons incident on the detector surface into electrical signals, and acquires and processes these electrical signals to construct an image of the internal features of the target 18. .

  The x-ray source 12 is controlled by a power supply / control circuit 24 that provides both power and control signals for the test sequence. Further, the detector 22 is coupled to a detector acquisition circuit 26 that commands acquisition of signals generated at the detector 22. The detector acquisition circuitry 26 can also perform various signal processing and filtering operations, such as dynamic range initial adjustment and digital interleaving.

  In the illustrated example embodiment, one or both of the power / control circuitry 24 and the detector acquisition circuitry 26 receive signals from the system controller 28. In some example systems, it may be desirable to move one or both of detector 22 or x-ray source 12. In such a system, there may also be a motor subsystem that performs this motion as a component of the system controller 28. In this example, the system controller 28 also includes signal processing circuitry, typically based on a general purpose or application specific digital computer. The system controller 28 may also include programs and routines executed by the computer, memory circuitry for storing configuration parameters, image data, and the like, interface circuits, and the like.

  There is also an image processing circuitry 30 in the illustrated embodiment of the x-ray imaging system 10. The image processing circuitry 30 receives the projection data acquired from the detector acquisition circuitry 26 and processes the acquired data to form one or more images based on x-ray attenuation.

  There are also one or more operator workstations 32 in the illustrated embodiment of the x-ray imaging system 10. The operator workstation 32 allows the operator to initiate and configure an x-ray imaging inspection and observe an image formed as part of the inspection. For example, the system controller 28 typically provides instructions or commands to the system controller 28 via one or more input devices associated with the operator workstation 32. Coupled to operator workstation 32.

  Similarly, the image processing circuitry 30 allows the operator workstation 32 to receive the output of the image processing circuitry 30 and display this output on an output device 34 such as a display or printer. Coupled to workstation 32. Output device 34 may include a standard or special purpose computer monitor and associated processing circuitry. In general, displays, printers, operator workstations, and similar devices provided within the system may be located locally with respect to the data acquisition component, or in a facility or hospital It may be located remotely from these components, such as elsewhere or a completely different location. Output devices and operator workstations located remotely from the data acquisition component can be coupled to the image acquisition system via one or more variable configuration networks, such as the Internet and virtual private networks. As will be appreciated by those skilled in the art, system controller 28, image processing circuitry 30 and operator workstation 32 are shown as separate from each other in FIG. 1, but these components are actually general purpose or specific. It may be embodied as a single processor type system such as a digital computer for application. Alternatively, some or all of these components are present in separate processor-based systems such as multiple general purpose or application specific digital computers configured to communicate with each other. Also good. For example, the image processing circuitry 30 may be a component of separate reconstruction and viewing workstations.

  In some embodiments of the invention, the detector 22 of the imaging system, such as that shown in FIG. 1, is constructed as a multilayer structure to obtain the desired electrical characteristics for the structure associated with each height (level). . In one example of an embodiment of the present invention, the exemplary detector 22 includes an array of photosensitive elements such as photodiodes formed of amorphous silicon disposed on a substrate. In such an embodiment, the photodiodes may be arranged as a row and column array defining pixels or pixels that are read by the detector acquisition circuitry 26. In this embodiment, each pixel includes a photodiode and a thin film transistor (TFT), which can be selectively activated using data lines and scan lines. Such an embodiment may also include a scintillator, which generates photophotons when illuminated with X-rays, and these photophotons are detected by a photodiode. Various embodiments of the present technique use vias to electrically couple data lines and / or scan lines to the respective electrodes of the TFT via various intervening layers. Data lines and / or scan lines connected in this manner are then connected to contact fingers or other conductive readout structures so that data can be transmitted from the sensor elements to the readout circuitry. As will be appreciated by those skilled in the art, vias are conductive structures that interconnect different conductive or metallized layers separated by one or more insulating layers. In this manner, electrical signals can be conducted between different layers or conductors of the multilayer structure.

  For example, in one embodiment of the present invention, the data line may be provided (offset) offset in the vertical direction from the source electrode and drain electrode present in the TFT used in the detector 22. For example, in some embodiments, the data lines and the source and drain electrodes are formed in separate processing steps, i.e. typically different conductive materials deposited at different times or at different stages of the formation process. It can be formed of layers. As a result, the layer of conductive material from which the data lines are formed can have a different thickness or composition than the layer of conductive material from which the source and drain electrodes are formed. For example, the data line can be formed of a layer thicker than the layer from which the source and drain electrodes are formed.

  An example of such an embodiment is illustrated in FIG. FIG. 2 shows a cross-sectional view of an exemplary TFT 36 in which the source electrode 38 and the drain electrode 40 are disposed on the semiconductor layer 42. In one embodiment, the semiconductor layer 42 includes amorphous silicon. In the illustrated embodiment, the data line 44, the source electrode 38, and the drain electrode 40 are formed in separate steps. In this embodiment, source electrode 38 and drain electrode 40 are formed by depositing a first layer of conductive material, such as molybdenum, chromium, aluminum, titanium, tungsten, or combinations thereof, on semiconductor layer 42. The In addition, in embodiments such as where the semiconductor layer 42 is made entirely or partially of amorphous silicon and the source and drain electrodes 38 and 40 are made entirely or partially of aluminum, the semiconductor layer 42 and Further, a blocking metal layer (not shown) may exist between the source electrode 38 and the layer that forms the drain electrode 40, and diffusion between the layers may be prevented. Further, the TFT 36 of the illustrated embodiment includes a gate electrode 46 disposed below the semiconductor layer 46. The gate electrode 46 is typically electrically isolated from the semiconductor layer 42 by a dielectric layer 48.

  The various layers of TFT 36 as described herein can be sputtering, chemical vapor deposition (CVD), plasma chemical vapor deposition (PECVD), radio frequency plasma chemical vapor deposition (RFPECVD), expanded thermal plasma. Chemical vapor deposition (ETPCVD), reactive sputtering, electron cyclotron resonance plasma chemical vapor deposition (ECRPECVD), inductively coupled plasma chemical vapor deposition (ICPECVD), sputter deposition, vapor deposition, atomic layer deposition (ALD) Or any suitable deposition technique such as a combination thereof. In addition, other types of deposition techniques suitable for fabricating integrated circuit or semiconductor based devices may be used to deposit some or all of the layers described herein.

  In one embodiment, source electrode 38 and drain electrode 40 are formed of a layer of conductive material having a thickness of about 0.02 microns to about 0.2 microns. The layer of conductive material is patterned to form the source electrode 38 and the drain electrode 40. In some embodiments, the layer of conductive material from which the source or drain electrode is formed is either before or after patterning the metal film to form the source electrode 38 and the drain electrode 40. A landing pad (not shown) for the via 50 can be formed. In these embodiments, the landing pad may or may not be disposed on the gate electrode 46. In some embodiments, forming the source electrode may also include electrically coupling the photosensitive element to the source electrode 38. In these embodiments, the source electrode 38 may be coupled to the photosensitive element via a second via (not shown) or by extending the source electrode 38 below the photosensitive element. It may be combined with.

  Source electrode 38 and drain electrode 40 are typically separated by channel 52. The bottom surface of the operative channel typically includes the exposed semiconductor material of the semiconductor layer 42. Channel 52 typically etches or removes a portion of the layer of conductive material disposed between source electrode 38 and drain electrode 40, exposing semiconductor layer 42, typically partially It is formed by etching. In one embodiment, channel 52 is about 1.5 microns to about 3 microns in length. When the source electrode 38 and the drain electrode 40 are formed of a relatively thin film of a conductive material or a metallized layer, the channel 52 having a relatively short length can be easily formed. As will be appreciated by those skilled in the art, shortening the channel length reduces charge retention in the channel 52, thereby reducing the noise level of the TFT.

  In the illustrated embodiment example, the TFT 36 also includes a dielectric layer 54 disposed over the source electrode 38 and the drain electrode 40. In this embodiment, the TFT 36 further includes a data line 44 formed of a second layer of conductive material disposed on the dielectric layer 54. In the approach of the present invention, the first layer of conductive material used to form the source electrode 38 and drain electrode 40 is thicker than the second layer of conductive material used to form the data line 44. And / or the composition may be different. For example, in one embodiment, the data line 44 is thickened, i.e., more than the source electrode 38 and the drain electrode 40, to reduce the resistance associated with the data line, thereby reducing the noise factor contributed by the data line. Also, it is desirable to make the data line 44 thick. In one example of such an embodiment, data line 44 is formed of a layer of conductive material having a thickness of about 0.2 microns to about 2 microns. In other embodiments, the data lines 44 include conductive elements such as molybdenum, aluminum, copper, titanium, tungsten, or combinations thereof.

  In addition, the TFT 36 includes a via 50 that electrically connects the data line 44 to the drain electrode 40. In one embodiment, via 50 is formed by cutting or selectively etching dielectric layer 54. In some embodiments, the common electrode and / or the TFT light shielding element can be formed in the layer from which the data line 44 is formed. In such an embodiment, a common electrode is provided between the scintillator and the connection to the photosensitive region, while a light shielding element is used to prevent light generated inside the scintillator from entering the TFT channel 52.

  FIG. 3 shows another example of the embodiment of the TFT 36 in which the gate electrode 46 is offset from the scanning line 58 in the vertical direction. For example, in some embodiments, the gate electrode 46 and the scan line 58 are formed in different processing steps, i.e., typically formed of layers of different conductive materials deposited at different times or stages of the formation process. obtain. Because the deposition steps are thus separate, the thickness or composition of each layer used to form the gate electrode 46 and the scan line 58 can be different. In other words, the layer of the conductive material that forms the scanning line may be different in thickness and / or composition from the layer that forms the gate electrode. For example, the scanning line 58 can be formed of a layer thicker than the layer from which the gate electrode is formed.

  FIG. 3 shows a cross-sectional view of TFT 56 according to one aspect of the technique of the present invention. The TFT 56 includes a gate electrode 46 disposed below the semiconductor layer 42. In one embodiment, the semiconductor layer 42 includes amorphous silicon. In one embodiment, the gate electrode 46 of the TFT 56 is formed by deposition of a first layer of conductive material such as molybdenum, chromium, aluminum, tungsten, titanium, or combinations thereof. Typically, in order to avoid scattered light from the inside of the substrate from entering the TFT 56, it is desirable to reduce the light transmittance value of the gate electrode. Thus, the minimum thickness of the gate electrode can be determined by the desired light transmittance value. In one embodiment, the light transmission value is less than 20%. In this embodiment, the thickness of the layer of conductive material forming the gate electrode is from about 0.02 microns to about 0.15 microns. In addition, when the thickness of the gate electrode 46 is minimized, the height of the structure of the TFT 56 is reduced. As will be appreciated by those skilled in the art, the higher the TFT 56, the greater the tendency of leakage current between the gate electrode 46 and either the source electrode 38 or the drain electrode 40. In addition, a via landing pad (not shown) for connecting the gate electrode 46 and the scanning line 58 may be formed during or after the formation of the gate electrode 46.

  In the illustrated embodiment, the semiconductor layer 42 and the gate electrode 46 are electrically isolated by a first dielectric layer, such as a dielectric layer 48. In this embodiment, the TFT 56 also includes a source electrode 38 and a drain electrode 40 disposed on the semiconductor layer 42.

  In one embodiment, a second dielectric layer, such as dielectric layer 54, is disposed on source electrode 38 and drain electrode 40. In addition, vias 60 can be formed in each TFT 56 of the detector 22 to expose the landing pad formed on the gate electrode 46. Typically, vias 60 are used to electrically couple gate electrode 46 and scan line 58. Further, in the illustrated embodiment, a scan line 58 is disposed on the dielectric layer 54. In one embodiment, scan line 58 is formed by depositing and patterning a second layer of conductive material on dielectric layer 54. In this embodiment, the scan line 58 is provided spatially offset from the channel 52, that is, the scan line 58 is provided generally offset from the channel 52 and therefore does not overlap the channel 52. As described above, in order to improve the performance of the detector 22, it is desirable to provide a low resistance scanning line. For example, in order for a flat panel detector to operate at a higher number of frames per second (60 fps), it is desirable to reduce the scan line resistance by increasing the scan line thickness. In one embodiment, the second layer of conductive material has a thickness of about 0.15 microns to about 1 micron. In other embodiments, the layer of conductive material from which the scan lines 58 are formed includes a metal such as molybdenum, chromium, aluminum, tungsten, titanium, etc., or combinations thereof. In addition, the layer of conductive material used to form the scan lines may also be patterned to form other structures such as vias and other inter structures. Further, in some embodiments, a dielectric layer such as dielectric layer 62 may be disposed over scan line 58.

  FIG. 4 shows a TFT 64 that incorporates a data line offset in the vertical direction from the source electrode 38 and the drain electrode 40 and a scan line offset in the vertical direction from the gate electrode 46 as described above. FIG. In the illustrated embodiment, the scan line 58 is disposed below the data line 44. In the illustrated embodiment, a dielectric layer such as dielectric layer 62 is disposed on the scan line 58. Further, in the illustrated embodiment, a dielectric layer such as a dielectric layer 66 is disposed on the data line 44. On the contrary, in other embodiments, the scanning line 58 may be disposed on the data line 44. In these embodiments, the scan line 58 may be disposed on a dielectric layer disposed on the data line, and the scan line 58 may be formed after the TFT 64 and the data line 44 are formed. The exemplary combined structure shown in FIG. 4 may be formed according to the structures discussed above with respect to FIGS. 2 and 3, and thus combine the respective advantages discussed with respect to each of the exemplary structures described above.

  While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. Therefore, it is to be understood that the claims are intended to cover all modifications and variations as fall within the spirit of the invention. Further, the reference numerals in the claims corresponding to the reference numerals in the drawings are merely used for easier understanding of the present invention, and are not intended to narrow the scope of the present invention. Absent. The matters described in the claims of the present application are incorporated into the specification and become a part of the description items of the specification.

1 is a diagram of an exemplary X-ray imaging system in accordance with an aspect of the present technique. 1 is a cross-sectional view of a thin film transistor having data lines located at different heights from a source electrode and a drain electrode according to an aspect of the present invention. FIG. 6 is a cross-sectional view of a thin film transistor having a scanning line located at a different height from the gate electrode according to an aspect of the present invention. FIG. 3 is a cross-sectional view of a thin film transistor having data lines and scanning lines located at different heights from a source electrode, a drain electrode and a gate electrode according to an aspect of the present invention.

Explanation of symbols

10 X-ray imaging system 16 X-ray radiation flow 20 Attenuated radiation 36, 56, 64 TFT
38 Source electrode 40 Drain electrode 42 Semiconductor layer 44 Data line 46 Gate electrode 48, 54, 62, 66 Dielectric layer 50, 60 Via 52 Channel 58 Scan line

Claims (10)

An electrode formed of a first layer of conductive material;
A readout line formed of a second layer of conductive material;
Vias electrically connecting the readout lines and the electrodes;
A detector (22) comprising:   The detector (22) of claim 1, wherein the first layer of conductive material and the second layer of conductive material differ in at least one of composition and thickness.   The detector of claim 1, wherein the first layer of conductive material comprises one of molybdenum, chromium, aluminum, tungsten, titanium, or combinations thereof. A plurality of detector elements each including a thin film transistor (36),
A source electrode (38) and a drain electrode (40) formed of a first layer of conductive material;
A data line (44) formed of a second layer of conductive material;
A via (50) electrically connecting the data line (44) and the drain electrode (40);
A detector for an imaging system comprising a plurality of detector elements including:   The detector according to claim 4, wherein the first layer of the conductive material and the second layer of the conductive material are different in at least one of composition and thickness.   5. The method of claim 4, further comprising a channel (52) disposed between the source electrode (38) and the drain (40) electrode and having a length of about 1.5 microns to about 3 microns. The detector described. A plurality of detector elements each including a thin film transistor (56),
A gate electrode (46) formed of a first layer of conductive material;
A scan line (58) formed of a second layer of conductive material;
A via (60) electrically connecting the scan line (58) and the gate electrode (46);
A detector for an imaging system comprising a plurality of detector elements including:   The detector of claim 7, wherein the first layer of conductive material and the second layer of conductive material differ in at least one of composition or thickness. A plurality of detector elements each including a thin film transistor (64),
A gate electrode (46) formed of a first layer of conductive material;
A source electrode (38) and a drain electrode (40) formed of a second layer of conductive material;
A scan line (58) formed of a third layer of conductive material;
A first via (60) electrically connecting the scan line (58) and the gate electrode (46);
A data line (44) formed of a fourth layer of conductive material;
A second via (50) electrically connecting the data line (44) and the drain electrode (40);
A detector for an imaging system comprising a plurality of detector elements including:   The one of the first layer of conductive material or the second layer of conductive material comprises one of molybdenum, chromium, aluminum, tungsten, titanium, or combinations thereof. Detector.

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